Mobility and a broadband have become development trends of modern communication technologies, and the 3rd Generation Partnership Project (3GPP) devotes itself to evolution of a Long Term Evolution (LTE) system as a 3G system and aims to evolve 3GPP radio access technologies toward high-data-rate, low-delay and optimized packet data applications.
For a higher peak rate, Single User Multi-Input Multi-Output (SU-MIMO) of Long Term Evolution Advanced (LTE-A) can support parallel transmission of at most 8 data layers. In order to support a flexible spatial pre-processing technology, a Demodulation Reference signal (DMRS)-based transmission scheme will be adopted over an LTE-A downlink.
During downlink MIMO transmission, an evolved NodeB (eNB) will assign each scheduled UE with a specific number of parallel data streams dependent upon a channel condition, a service characteristic, a priority and other factors of each UE, where each data stream is referred to as a layer. In order to support the Spatial Multiplexing technology of at most 8 layers, an 8-port DMRS will be defined in the LTE-A. The DMRS is processed with the same pre-coding as data, and the UE can know an equivalent channel matrix after pre-coding and demodulate the data by measuring the DMRS. The downlink SU-MIMO of the LTE-A can support at most 8 layers, each of which corresponds to a DMRS port. A process flow of a downlink physical channel is as illustrated in FIG. 1.
As illustrated in FIG. 1, downlink data transmission of each UE can support at most 2 codewords, each of which corresponds to a Transport Block (TB), and a Modulation and Coding Scheme (MCS) of each codeword can be controlled by an eNB dependent upon a link condition. The eNB will further determine the number of data streams to be used for parallel transmission dependent upon a spatial characteristic of a channel, a service type, a priority, the amount of traffic and other factors. Transmission of at most 8 layers can be supported in the LTE-A. A codeword-to-layer mapping relationship is defined by a layer mapper module, and at present an agreement has been reached upon definition of layer mapper function in spatially multiplexed transmission, where specific layer mapper functions are as depicted in Table 1 and Table 2.
TABLE 1Codeword-to-layer mapping (Rank = 1-4)The numberThe number ofCodeword-to-layer mappingof layerscodewordsi = 0, 1, . . . , Msymblayer − 111x(0)(i) = d(0)(i)Msymblayer = Msymb(0)22x(0)(i) = d(0)(i)Msymblayer = Msymb(0) = Msymb(1)x(1)(i) = d(1)(i)21x(0)(i) = d(0)(2i)Msymblayer = Msymb(0)/2x(1)(i) = d(0)(2i + 1)32x(0)(i) = d(0)(i)Msymblayer = Msymb(0) = Msymb(1)/2x(1)(i) = d(1)(2i)x(2)(i) = d(1)(2i + 1)42x(0)(i) = d(0)(2i)Msymblayer = Msymb(0)/2 = Msymb(1)/2x(1)(i) = d(0)(2i + 1)x(2)(i) = d(1)(2i)x(3)(i) = d(1)(2i + 1)
TABLE 2Codeword-to-layer mapping (Rank = 5-8 and mapping of single-codeword to layers 3-4)The numberThe number ofCodeword-to-layer mappingof layerscodewordsi = 0, 1, . . . , Msymblayer − 131x(0)(i) = d(0)(3i)Msymblayer = Msymb(0)/3x(1)(i) = d(0)(3i + 1)x(2)(i) = d(0)(3i + 2)41x(0)(i) = d(0)(4i)Msymblayer = Msymb(0)/4x(1)(i) = d(0)(4i + 1)x(2)(i) = d(0)(4i + 2)x(3)(i) = d(0)(4i + 3)52x(0)(i) = d(0)(2i)Msymblayer = Msymb(0)/2 = Msymb(1)/3x(1)(i) = d(0)(2i + 1)x(2)(i) = d(1)(3i)x(3)(i) = d(1)(3i + 1)x(4)(i) = d(1)(3i + 2)62x(0)(i) = d(0)(3i)Msymblayer = Msymb(0)/3 = Msymb(1)/3x(1)(i) = d(0)(3i + 1)x(2)(i) = d(0)(3i + 2)x(3)(i) = d(1)(3i)x(4)(i) = d(1)(3i + 1)x(5)(i) = d(1)(3i + 2)72x(0)(i) = d(0)(3i)Msymblayer = Msymb(0)/3 = Msymb(1)/4x(1)(i) = d(0)(3i + 1)x(2)(i) = d(0)(3i + 2)x(3)(i) = d(1)(4i)x(4)(i) = d(1)(4i + 1)x(5)(i) = d(1)(4i + 2)x(6)(i) = d(1)(4i + 3)82x(0)(i) = d(0)(4i)Msymblayer = Msymb(0)/4 = Msymb(1)/4x(1)(i) = d(0)(4i + 1)x(2)(i) = d(0)(4i + 2)x(3)(i) = d(0)(4i + 3)x(4)(i) = d(1)(4i)x(5)(i) = d(1)(4i + 1)x(6)(i) = d(1)(4i + 2)x(7)(i) = d(1)(4i + 3)
For public pilot-based transmission and demodulation schemes in the Rel-8/Rel-9, a pre-coding module performs pre-coding based upon a Pre-coding Matrix Indicator (PMI). For the DMRS-based transmission scheme, the pre-coding module functions to map respective data layers to DMRS ports. In the transmission mode 7 of the Rel-8, layer-to-port mapping is just simple one-to-one mapping because there is only one dedicated pilot, the port 5. In the transmission mode 8 of the Rel-9, two dedicated pilots, the port 7 and the port 8, are defined, and a layer-to-port mapping scheme is defined as
      [                                                      y                              (                7                )                                      ⁡                          (              i              )                                                                                      y                              (                8                )                                      ⁡                          (              i              )                                            ]    =            [                                                                  x                                  (                  0                  )                                            ⁡                              (                i                )                                                                                                        x                                  (                  1                  )                                            ⁡                              (                i                )                                                        ]        .  
A DMRS of at most 8 ports is supported in the LTE-A, and FIG. 2-1 illustrates DMRS patterns at the ranks=1-4 as already determined at present in the LTE-A. DRMS patterns at the ranks 5-8 are under ongoing discussion, but substantially it can be determined that in the DMRS patterns at the ranks 5-8, respective DMRS ports will be multiplexed in a Code Division Multiplexing-Frequency Division Multiplexing scheme. FIG. 2-2 illustrates an example of the DMRS patterns at the ranks=5-8.
As opposed to the Rel-8/Rel-9, the layer-to-port mapping relationship in the LTE-A is relatively complex in that both a scheme of allocating layers corresponding to respective codewords across respective CDM groups and a relationship between a DMRS pattern and a rate matching module during retransmission have to be taken into account. The method illustrated in FIG. 3 can be adopted at present for the issue of layer-to-port mapping in the LTE-A. Under a mapping principle thereof, layers corresponding to different codewords have to be mapped into different CDM groups.
However the foregoing mapping solution has the following drawbacks:
(1) At the rank=2, two codewords have to be mapped respectively into different CDM groups under the mapping rule thereof, and this mapping scheme is incompatible with the transmission mode 8 of the Rel-9, so it is inconvenient to schedule an Rel-9 UE and an Rel-10 UE jointly;
(2) At the rank=2, two codewords have to be mapped respectively into different CDM groups, and therefore a DMRS overhead is increased from 12 pairs of Resource Elements (REs)/Physical Resource Blocks (PRBs) to 24 pairs of REs/PRBs;
(3) If it is ensured under the principle of the existing solution that a layer corresponding to each codeword can only be put into one CDM group with a DRMS overhead of 12 pairs of REs/PRBs, then only transmission of a single codeword can be supported in transmission at the rank=2; and
(4) No method of port mapping in retransmission has been considered in the existing solution.
Therefore it is desired to propose a working solution to mapping to a demodulation reference signal port while being compatible with an existing system and keeping a lower demodulation reference signal overhead.